Altitude sensing in aircraft and rockets has been achieved in the past in several ways. For example, barometric altimeters have been used, as have radar based systems. It is also possible to use data from an inertial or other navigation system relating to vehicle movements and to calculate or estimate the altitude from that data.
It has been proposed to provide a defence against a self-guided target seeking missile by means of a decoy rocket which is launched upon detecting an approaching missile. It is proposed that the decoy rocket slowly move away from the target such as a ship while at the same time emitting strong signals intended to be detected by the approaching missile so that the missile will guide itself towards the decoy rocket rather than the target. The flight control unit for such a decoy rocket is desirably self-contained, that is there will be no direct control from the launch site. The flight control unit will be operable to gather data such as the height, movement direction, speed and attitude of the rocket, to determine the desired flight path of the rocket and to carry out the flight control operations.
Since the decoy rocket is intended to be expendable, the component parts should therefore be relativey inexpensive. The simplest altitude determining system known and expected to provide suitable accuracy for such a rocket would be based on barometric pressure sensing. However, the decoy rocket is intended to be movable generally horizontally in any direction without the direction of movement being accurately known or even approximately known in relation to the configuration of the rocket itself. That is, the direction of movement of the rocket in relation to the configuration of the rocket will not be predetermined in advance or will not be predetermined for the remainder of the flight after launch.
One problem with measuring the rocket altitude based on barometric pressure measurement partially results from the configuration and nature of movement of the rocket. For example, it is expected that the decoy rocket will be generally cylindrical and be about seven feet long and about six inches in diameter. The rocket will be launched and the axis of the cylindrical body is expected to be generally vertical. The rocket is expected to have a zero or small vertical speed so that it can hover or rise slowly and also move generally horizontally to attract the approaching missile from its target. For the flight control unit of the decoy rocket to effectively operate, the altitude and vertical speed of the rocket must be known at least approximately.
With this type of rocket fluid pressure differentials will be created around the rocket body surface as the body moves horizontally relative to the fluid, i.e. with a component of velocity of the body being transverse to the vertical longitudinal axis of the body. For example, in the case of the generally cylindrical body of a decoy rocket body which is arranged with its longitudinal axis generally vertical and moving in air in a horizontal direction, the portion of the body surface which is moving in a forward direction relative to the air will experience a positive net air pressure while the two "sides" of the cylindrical body will each experience a negative net pressure due to the air flow over those side surfaces and the "back" surface will also experience a negative net pressure--see FIG. 1 of the accompanying drawings where the pressure distribution curve P around a cylindrical surface moving in a field is schematically illustrated. The magitude of the net pressure at any point on the surface of the body, at least on a relative scale, may be estimated from the pressure distribution curve P in FIG. 1 by measuring the length of a line drawn radially from the desired point on the body surface to the pressure distribution curve P.
Because the net pressures at the body surface vary from ambient pressure depending on the speed and direction of relative horizontal movement, it will not be possible to reliably and accurately determine the ambient barometric pressure simply by taking an air pressure reading at one point of the body surface. Also, because the direction of relative body movement will not be known in relation to the body configuration, it will not be possible to determine ambient air pressure by locating a pressure sensor at one of the two points of the body surface where the net air pressure is zero, i.e. where the absolute pressure equals the ambient static air pressure.
Altitude sensing or determination for the rocket using signals received from the launch site are also expected to be unsuitable, because these signals may be subject to accidental or deliberate interference.